The present invention relates to a sheet feeding device having an air sheet feeding device for feeding a sheet by sucking the sheet by the suction force of air, and an image forming system including the aforementioned sheet feeding device connected to an image forming apparatus.
There is a sheet feeding device that connects to such an image forming apparatus for forming an image on a sheet as a photocopier or a laser beam printer and supplies a sheet to the image forming apparatus.
The sheet feeding device has a sheet storage section for storing a great number of sheets. Synchronized with the image forming operation of the image forming apparatus, the sheet feeding device feeds out sheets one by one to the image forming apparatus. If a sheet is not picked up from the sheet storage section, a feeding error occurs. Also, if a number of sheets are picked up in one operation, a multiple sheet feed occurs, with the result that a paper jam or similar failure occurs.
One of the techniques known in the conventional art is a sheet feeding device (hereinafter referred to as “air sheet feeding device”), wherein, when sheets are picked up from the sheet storage section one by one and are supplied to the image forming apparatus, the suction force of air is used to make the sheets to be sucked on a conveyance belt, which are then conveyed driven by the conveyance belt.
The conventionally known roller sheet feed device using the frictional force of an elastic roller such as a rubber roller has a disadvantage in that its conveying force greatly depends on the chronological deterioration of the elastic roller, foreign substances attached to the surface of the elastic roller, or properties of the sheets to be conveyed. By contrast, the air sheet feeding device has an advantage in that stable conveying force can be maintained for a long period of time regardless of the sheet properties.
In the air sheet feeding device, air is blown from the periphery side of the sheets stored in the sheet storage section. The air blown from the periphery side makes its way into the gap between sheets to float and separate the sheets. Then, the floated sheet at the topmost position made to be sucked on the conveyance belt arranged above the sheet storage section and conveyed, so that sheets stored in the sheet storage section are picked up one by one and are conveyed.
One of the proposals made in recent years is an air sheet feeding device wherein air is blown from the forward position in the conveyance direction of the sheet stored in the sheet storage section and from the side perpendicular to the conveyance direction (side in the direction of width) so that the sheet stored in the sheet storage section is floated (refer to Japanese Patent Application Publication No. 2009-227378).
Incidentally, a curl may occur to the sheet. If the sheet absorbs moisture, dimensions will increase. If a moistened sheet is placed in an environment of low humidity, the moisture content is discharged and the sheet shrinks. If discharge of moisture is not uniform, the sheet will be deformed without uniform shrinkage, and will be curled.
If the sheets stored in the sheet storage section are curled and air is blown to the sheets, air fails to proceed between sheets, with the result that successful floating and separation of the sheets cannot be achieved.
Another proposal made in recent years is an air sheet feeding device wherein, when the sheets stored in the sheet storage section have their leading edges curled upward or downward in the conveyance direction, pressure is applied to the sides of the sheets stored in the sheet storage section so that the curl on the leading edges in the conveyance direction is corrected, and sheets can be floated and separated successfully (Japanese Patent Application Publication No. H09-309624).
The air sheet feeding device disclosed in the Japanese Patent Application Publication No. H09-309624 operates in the following manner to pick up the sheets stored in the sheet storage section and feeds these sheets one by one.
In the first place, pressure is applied to the sheets stored in the sheet storage section by a pressure means to form a curl where the cross section perpendicular to the sheet conveyance direction is convexed upward. By forming such a curl where the cross section perpendicular to the conveyance direction is convexed upward, the upward or downward curl on the side of the leading edge of the conveyance direction is corrected.
This is followed by the step of blowing air from the leading edge side of the sheet in the conveyance direction. Since the curl that was present on the side of leading edge in the conveyance direction has been corrected, the leading edge of the sheet is kept in parallel with air flow. This makes easy for air to come between sheets, with the result that sheets are floated.
Then, the topmost sheet out of the floated sheets is made to be sucked on the conveyance belt. After the pressure applied by the pressure means has been released, the sheet is conveyed.
In the air sheet feeding device proposed in the Japanese Patent Application Publication No. H09-309624 characterized by the aforementioned operation principle, pressure is applied to the sheets stored in the sheet storage section by a pressure means to ensure that the sheets stored in the sheet storage section will form a curl where the cross section perpendicular to the sheet conveyance direction is convexed upward. This causes a close adherence between sheets. Thus, the air flow for floating fails to enter the gap between sheets sufficiently, and floating of the sheets is practically difficult. It is also practically difficult to make the sheets pressed by the pressure means to be sucked on the conveyance belt. Further, a complicated structure must be designed because the pressure means has to be provided with a moving mechanism for pressing and separating the sheets.
In the air sheet feeding device disclosed in the Japanese Patent Application Publication No. 2009-227378, by contrast, air is blown from the side perpendicular to the conveyance direction (side in the direction of width). This makes it possible to float the sheets curled in such a way that the leading edge of the sheets in the conveyance direction is curled upward or downward, independently of the effect of the curl, when the sheets stored in the sheet storage section are to be floated. However, when the sheets stored in the sheet storage section is convexed downwardly in the direction perpendicular to sheet feed, the sheets are made to float easily by the air blown from the side (side in the direction of width). Thus, a plurality of sheets will be floated, with the result that a plurality of sheets, together with the floating topmost sheet, may be adhered to the conveyance belt and may be conveyed together.
The object of the present invention is to provide a sheet feeding device provided with an air sheet feeding device of simple structure capable of picking up sheets one by one with certainty and conveying these sheets, without being affected by a curl of sheets stored in the sheet storage section.